1. Field of the Invention
The present invention relates to structure of a magnetic recording medium such as a magnetic disc, and particularly controls surface roughness in a final form by means of technique concerning surface treatment of a overcoat of a magnetic recording medium corresponding to ultra-low floating of a magnetic head.
2. Description of the Related Art
Generally, the magnetic storage device is, as shown in FIG. 1, comprised of a magnetic recording medium 1, a spindle 2 for holding and rotating it, a magnetic head 3, which performs reading and writing, a servomechanism 4 for positioning the magnetic head, and an electric circuit 5 for driving them, and these are combined together to constitute one magnetic disc drive.
Also, the magnetic recording medium is generally comprised of a metallic undercoat layer, a magnetic layer, a protective layer, and a lubricating film layer on a non-magnetic substrate, and for characteristic properties required for the magnetic recording medium, not only R/W characteristics, but also matters concerning resistance to sliding property and the like such as damages on a surface of the medium by the contact with a magnetic head become important. Particularly, as regards a surface of the substrate, there is generally used a method for preventing the magnetic head from adhering to a magnetic disc by forming a fine groove by circumferential or non-oriented grinding called xe2x80x9ctexturexe2x80x9d due to a machining method using fine abrasive grains.
Also, as other methods, there are a method (method called xe2x80x9cdepotexturexe2x80x9d) in which a similar effect to the texture is obtained by forming a substrate or a magnetic layer with a fine projection on the surface thereof by means of sputtering, and a method (etching texture method) for obtaining the similar effect by, after formation of a overcoat, coating with Teflon particles or the like as masking material, etching the surface by a dry etching method, and machining the surface of the protective layer to form irregularities on the protective layer itself, and the like.
The object of either method is mainly to prevent the magnetic head from sticking onto the magnetic disc, and to confirm reliability by means of contact start stop (hereinafter, referred to as xe2x80x9cCS/Sxe2x80x9d). In recent years, however, in these days in which as particularly the recording density becomes higher, flying height of the magnetic head has reduced less than 10 nm, a lamp load system (method for placing the magnetic head on the magnetic disc after the magnetic disc is kept rotated) has mainly been used in place of the CS/S system, and it has become impossible to represent a durability of a data surface with which the actual magnetic head comes into contact irregularly while floating by means of a conventional evaluation method for CS/S durability, adhesive force and the like.
In the conventional technique, there has been generally used a method for reducing the damage caused by a shock at the time of contact by providing the data surface with a fine groove called xe2x80x9ctexturexe2x80x9d (hereinafter, referred to as TEX) by machining work, and for preventing the magnetic head from sticking using lubricant. According to this method, TEX machining itself means to provide the substrate with surface roughness, and if the surface roughness becomes large, the amount of floating of the magnetic head becomes higher inevitably, and a head take-off height (Hto), which is a minimum height for the magnetic head to float from the surface of the disc, also becomes higher. Therefore, it cannot withstand the use in the extra-low floating area. Further, if the substrate has surface roughness to some extent, it will be emphasized as it is, or more than it to appear on the surface roughness of the under film, the magnetic film, and the protective layer to be laminated naturally on the substrate, and the surface roughness of the magnetic recording medium finally completed will become equal to or higher than that of the substrate. Thus, the surface of the substrate for use is required to be made as flat as possible, and for this reason, the characteristics of resistance to sliding properties to be required for the protective layer and the lubricating film will require to have as much durability as possible. Also in order to solve these problems, such high-hardness, high-strength protective layer as diamond-like carbon film (amorphous hard carbon hydride film)(in the present specification, described as DLC film) is adapted to be required.
For a process for the DLC film, however, the CVD system, PE(Plasma Enhanced)CVD system, IBD(Ion Beam Deposition) system, and others have been used. The DLC film formed by means of these system has features in which the covering rate of the film is very high, the surface is made ultra-flat, and free radicals and the like which connect to a functional group of the lubricant become less. When such a surface is coated with lubricant, the lubricant is difficult to adhere, and is easy to scatter. Also, when the magnetic head comes into contact with the ultra-flat surface, a tangential force at the time of contact becomes great. The flatter the surface becomes, the easier this occurs, which causes the crash.
Further, the protective layer thickness to be required becomes equal to or less than 5 nm as the recording density becomes higher. According to the prior art, in order to enhance the strength of the protective layer, a nitrogen ion has been implanted into the DLC film by means of the ion implantation in Japanese Patent Laid-Open Application No. 1-263912. According to the Laid-Open Application, a nitrogen ion implantation layer is formed on the surface of the DLC film thereby, and the hardness of the film becomes higher toward the surface layer. In this Laid-Open Application, however, as the condition for the ion implantation, the nitrogen ion is accelerated at 5 keV to 60 keV. When accelerated under this condition, it reaches to a depth of about 270 A to 3600 A, very high acceleration is given to the overcoat thickness being 100 to 200 xc3x85, and even though an etching phenomenon occurs before the implantation, it is a phenomenon quite in order. Further, since it is assumed that there is no spacing loss on the surface obtained as described above, and since the nitrogen ion accelerated actually reaches the magnetic layer, it is evident that the magnetic recording characteristics of the magnetic layer will be certainly affected, and it is considered that any increase in error rate due to dropout cannot be expected.
These problems have a similar result even if the method specified in Japanese Patent Laid-Open Application No. 9-219020 is employed, and in a portion as thick as 100 to 200 xc3x85 in overcoat thickness, depth of implantation may be controlled by reviewing the implantation condition. Under present circumstances, however, the protective layer thickness is equal to or less than 50 xc3x85, and it becomes difficult by means of these methods. Further, the nitrogen ion entered by implementation bonds to carbon, which is a basic composition of the DLC film, on rare occasion, and is none other than one physically entered.
Therefore, it can be seen that any improvement effects of such adhesive power of lubricant onto the DLC film as described above cannot be expected either. Also, since these methods are unable to control the form of the surface of the DLC film (it has been specified that the form does not change), the tangential force of the magnetic head at the time of contact is steadily rising, and any improvement in the tribological resistance to sliding operation is not wished.
Thus, in the present invention, there has been proposed a technique and a method for treating only an ultra-surface layer of the DLC film equal to or less than 5 nm with the aim of arbitrarily forming surface roughness on a DLC film, which is an exceedingly thin film of 5 nm or less, at surface roughness Ra of the substrate being 0.4 nm or less with an amount of floating of the magnetic head being 10 nm or less, that could not be achieved by the above-described prior art, and ensuring the durability to sliding operation in the ultra-low floating area with lubricant being easily deposited on the surface of the DLC film.
Since the conventional method using the ion implantation can be applied only to the high-voltage and exceedingly small current condition for increasing the efficiency in high vacuum under the feature of the gun, it has been decided in the present invention to pursue the study using an ion beam gun, that has been studied to form the DLC film in recent years.
In order to effectively prepare the DLC film, these ion beam guns prepare hard DLC films by decomposing gas such as ethylene, methane and acetylene which become raw material, with plasma for ionization, and accelerating by applying energy of about 200 eV onto the substrate. It is possible to arbitrarily control accelerating voltage, and to enter to depth of about 10 xc3x85 at 150 eV for an ultra-thin overcoat of 5 nm or less, and has features that controllability in the depthwise direction is good, it is possible to raise the ion current density to about 5 mA/cm2 at lower voltage than the ion implantation, and the throughput efficiency is high. In this connection, in the ion implantation, several tens kV and 1 mA/cm2 at maximum are applied, and voltage is increased instead of being unable to increase the current density.
In the present invention, as gas to be used for treatment, there has been adopted nitrogen. As the reason, it has been considered that on a surface having a CN bond, fluorine lubricant is prone to adhere, and that since the atomic radius of nitrogen is as small as 0.53 xc3x85 as compared with 1.91 xc3x85 of Ar, or the like, it is possible to prevent the sputtering phenomenon, that is, the protective layer from being damaged, and to reform the surface while particularly making the most of a feature of the DLC film being rigid.
Basic data actually obtained are shown in FIGS. 2 and 3. FIG. 2 shows a Raman spectrum of the DLC film on the surface of the disk prepared by treatment according to the present invention. In such a method having great shock as the conventional one, it can be seen that the original film quality of the DLC film is not maintained, but the film itself has been changed. In the treatment according to the present invention, however, it can be seen that the spectrum of the DLC film itself is not changed. FIG. 3 shows a result obtained by measuring an amount of nitrogen in the DLC film of a similar sample in the depth-wise direction through the use of an ESCA (ELECTRON SPECTROSCOPY FOR CHEMICAL ANALYSIS).
The measuring apparatus and measuring conditions are shown below.
Measuring apparatus: QUANTUM 2000 (SCANNING ELECTRON SPECTROSCOPY FOR CHEMICAL ANALYSISS MICROPROBE SYSTEM) manufactured by PHYSICAL ELECTRONICS, INC.
X-rays generating condition: Beam diameter 200 xcexcm-44W, Al target and monochrome X-rays (AlKxcex1) have been used as an excitation source.
Analysis area mode: Point analysis
Diameter of beam for use: 200 xcexcm"PHgr"
Analyzer condition: Analysis mode MULTIPLEX, pass energy 117.4 eV, step size 0.125 eV
Angle of ejection: angular resolution; quantitative calculation from a narrow spectrum has been performed at TAKE OFF=6 to 75xc2x0.
FIG. 3 shows one example of the result of the ESCA analysis. The abscissa axis indicates film thickness from the surface (0) of the DLC film, and the ordinates axis indicates changes in nitrogen density and carbon density in the depth-wise direction. In this case, the carbon density increases toward the depth direction from the surface, the nitrogen density shifts at substantially constant density from the surface to the depth of 10 xc3x85, and as the entered depth, it can be observed to about 13 xc3x85, which is a position where the carbon density and the nitrogen density intersect. In the case of accelerating voltage being 150 eV, since entry to the depth of about 10 xc3x85 is calculated from the simulation, this result substantially coincides with the calculated value.
From the foregoing, it can be also seen clearly that the amount of nitrogen more increases toward the ultra-surface layer according to the present invention. The accelerating voltage at this time is 200V in setting(about 150 eV), and from these data, it can be seen that the depth of penetration of N is about 10 xc3x85.
Further, in order to observe the form of the surface, observation using an atomic force microscope (AFM) has been performed, and changes in the surface roughness are shown in FIG. 4. From this figure, it could be confirmed that it is possible to arbitrarily change the original roughness of the DLC surface according to the present invention.
In this connection, in this measurement, the surface roughness has been calculated from the surface form obtained by measuring through the use of an interatomic power microscope NANOSCOPE [II] manufactured by Digital Instruments, Inc. As an index for the surface roughness here, numerical values of roughness to be defined in the following definition have been used. For details, please refer to their instruction manual.
The average value: Ra of a surface on which the center plane is made as the reference is represented by a numerical formula 1, and in this case, f(x, y) has the center plane made as the reference, and a surface Lx and Ly represents dimension of the surface. Numerical Formula 1      R    ⁢          xe2x80x83        ⁢    a    =            F      ⁡              (                              1            ·            L                    ⁢                      xe2x80x83                    ⁢          x          ⁢                      xe2x80x83                    ⁢          L          ⁢                      xe2x80x83                    ⁢          y                )              [                  ∫        0                  L          ⁢                      xe2x80x83                    ⁢          y                    ⁢                        (                                    ∫              0                              L                ⁢                                  xe2x80x83                                ⁢                x                                      ⁢                          |                              f                ⁡                                  (                                      x                    ,                    y                                    )                                            |                              ⅆ                x                                              )                ⁢                  ⅆ                      y            :                              
As shown in the definition of Ra, defined in the description of the AFM, the surface according to the present invention has been set to 0 in the direction of the depth of the surface, that is, the top surface having the center plane to be defined by the surface roughness Ra described previously.
In this connection, the measuring area has been set to 2.5 xcexcm square.